1. Field of the Invention
The present invention relates to a hall-type electric propulsion, and more particularly to a hall-type electric propulsion that realizes both overheating protection and operational stability, thereby simultaneously solving the problem of waste heat which worsens with micronization and the problem of discharge current oscillation.
2. Description of the Related Art
In space propulsion systems, various functions including spacecraft station keeping and orbit correction are required, and therefore various propulsion systems covering a wide output range more than kN levels from mN levels are required. At the same time, functions such as minimization of impulse-bit, high-responsiveness, and lifespan extension have come to be required as a result of mission diversification. At present, chemical propulsion systems using hydrazine are used in apogee motors, station keeping thrusters and so on, while electric propulsion systems are used mainly to control the station and orbit of geo-stationary satellites. Since electric propulsion is high-specific impulse and low-thrust, and the dry weight of the power supply and so on is large, electric propulsion is particularly effective in missions requiring a large speed increment. As for missions requiring an extremely large speed increment, in many cases the only propulsion system capable of realizing such missions at present is electric propulsion. As electric propulsion becomes commercially viable, it has become important not only to improve the propulsion performance, but also to provide system interfaces superior to those of conventional propulsion units, which are problematic in terms of the optimization of plume plasma shape, electromagnetic interference, contamination, waste heat due to large power devices and so on.
An electric propulsion is a space propulsion that converts sunlight energy or the like into electric energy, uses the electric energy to turn a propellant into plasma through various methods, accelerates the generated plasma in various forms, and generates thrust from the resulting reaction. Electric propulsions can be largely divided into three types, namely an electrostatic acceleration type, an aero-thermal acceleration type, and an electromagnetic acceleration type, in accordance with differences in the thrust generation mechanism.
An ion engine, representing the electrostatic acceleration type, generates plasma through direct current discharge or the like, and obtains thrust by accelerating and injecting ions in the generated plasma using an electrostatic field (of approximately 1,000V) applied between porous grid. A considerably higher specific impulse (between 2,000 and 7,000 seconds) than that of a chemical propulsion can be achieved with high efficiency (up to 80%), but the thrust density is comparatively small (thrust=several mN to 200 mN) due to the restrictions of the space-charge limited current rule, and in the low specific impulse range, propulsion efficiency deteriorates dramatically. Several types of plasma generation methods, including an RF-type method, have been proposed.
A thrust generation mechanism of an arc jet-type electric propulsion, which serves as an aero-thermal acceleration-type propulsion, subjects a propellant to ionization and Joule heating through an arc discharge formed between a rod-shaped cathode and a ring-shaped anode disposed coaxially with the rod-shaped cathode, and then expands and accelerates the heated plasma using a supersonic nozzle. High thrust density (thrust=150 mN to 2N) is obtained, but heat loss onto the wall is large, and therefore the propulsion efficiency is low (30 to 40%) in comparison with an electrostatic acceleration-type propulsion, and the specific impulse (between 500 and 2,000 seconds) is not especially high. As regards commercial viability, the following important problems remain unsolved: (1) cathode wear, which determines durability, reaches 5 μg/C during a steady state operation, and this wear must be reduced; (2) heat loss must be improved.
An MPD (Magneto-Plasma-Dynamic)-type electric propulsion, which is a propulsion representing the electromagnetic acceleration type, has a similar basic structure to the arc jet-type electric propulsion. The propellant is heated and turned into plasma by arc discharge, whereupon a high discharge current in the order of kA is caused to flow between electrodes to induce a magnetic field in a circumferential direction. The generated plasma is accelerated in an axial direction by a Lorentz force, which is the interaction between the induced magnetic field and the current, and as a result, thrust is obtained. A feature of the MPD-type electric propulsion is that it obtains the highest thrust (up to 10N) of all electric propulsions, and is therefore promising as a propulsion for interplanetary navigation of the future. The obtained specific impulse has a wide range of approximately 1,000 to 6,000 s, but at present, the typical propulsion efficiency of approximately 10 to 50% remains low.
Finally, the hall-type electric propulsion according to the present invention will be described. As shown in FIG. 6, a hall-type electric propulsion has a ring-shaped, axisymmetrical acceleration channel 505 that turns a neutral particle (propellant) 503 introduced through an anode hole 502 into plasma and accelerates a generated ion 504. When a length Ld of the acceleration channel is designed to be shorter than the ion cyclotron radius and longer than the cyclotron radius of an electron 506 (which is emitted from a cathode 507 and caused to flow in reverse through the acceleration channel in an anode direction), an electron 510 is subjected to E×B drift in the circumferential direction by the interaction between an axial electric field E and a radial external magnetic field B, whereby a “hall current (the name of which is derived from the hall-type electric propulsion)” is induced. By accelerating the ion 504 using an electric field generated through the electromagnetic interaction between the hall current and the externally applied magnetic field B, the hall-type electric propulsion acts in an identical manner to the “electrostatic acceleration type”, and yet the hall-type electric propulsion also shares features with the “electromagnetic acceleration type” in that the accelerated ion 504 is neutralized using an electron 513 from the cathode and a high thrust density is obtained regardless of the space charge limited current rule by maintaining the quasi-neutrality of the acceleration-zone (the acceleration mechanism will be described in further detail below). Hence, in principle, a high specific impulse (up to 3,000 s), a high thrust efficiency (70%) and a high thrust density (up to 1.5N) are all achievable (see Japanese Unexamined Patent Application Publication H7-71361 and Japanese Unexamined Patent Application Publication 2006-125236, for example).
The discharge characteristic (current-voltage characteristic) of the hall-type electric propulsion is divided into two operating modes, namely a “high voltage mode” and a “low voltage mode”. An operating mode in which the discharge current increases dramatically when the discharge voltage is raised is known as a “low voltage mode”. The discharge current is the product of charge density and velocity, but in the operating range of the low voltage mode, the degree of propellant ionization in the acceleration channel is low, and therefore, when the discharge voltage is raised to promote propellant ionization, the charge density increases, leading to an increase in the discharge current. Meanwhile, when the discharge voltage is raised further, the operating mode shifts to the “high voltage mode”, in which the discharge current increases more gently relative to increases in the discharge voltage. The reason for this is that since the propellant is already fully ionized in the high voltage mode, further current increases are not complemented by charge increases due to ionization, and therefore the current increases must be complemented by increases in ion velocity, which serves as another current increasing element, alone. The point at which the discharge current increase varies dramatically is known as the “knee point”, and the current value at that time is known as the “knee current”. Since the knee current is highly dependent on the discharge current amount when the propellant is completely ionized, the Knee current decreases as the flow rate of the propellant decreases.
With respect to thrust generation, one problem of the hall-type electric propulsion is a discharge current oscillation phenomenon, which is observed during an operation in the high voltage mode (as described above, in a region of the discharge characteristic at and above the “knee point”, where the discharge current substantially stops varying relative to the discharge voltage), which is the normal operating mode of a hall-type electric propulsion. Discharge current oscillation causes reductions in the propulsion performance and durability as well as operational instability, and in order to respond to space missions requiring a high reliability for a long period and a long lifespan, it is vital to learn the physical mechanisms of discharge current oscillation and establish design guidelines for solving it. Low-frequency discharge current oscillation in the 20 kHz-range, which is particularly prevalent during a high voltage mode operation, has the greatest amplitude of the various coexisting oscillation components, and as the discharge voltage increases, the discharge current shifts from oscillation to instability such that finally, it becomes impossible to maintain discharge, and the operation will be halted.
In discharge current oscillation, various oscillation components coexist over a wide frequency band range extending from kHz to MHz. The oscillation components have been classified into the following five frequency bands using the frequency order and oscillation characteristic as references.
1.Ionization Oscillation:104 to 105Hz2.Transit-time Oscillation:105 to 106Hz3.Electron-drift Oscillation:106 to 107Hz4.Electron-cyclotron Oscillation:109Hz5.Langmuir Oscillation:108 to 1010Hz
Of these five types of oscillation, the first three occur particularly strikingly during an operation of a hall-type electric propulsion, while GHz order-oscillation of the fourth and fifth types is unique to plasma and therefore considered unavoidable. Low-frequency discharge current oscillation in the 20 kHz-range has the greatest amplitude of the various coexisting oscillation components and leads directly to operational instability, and is therefore of particular importance with respect to the propulsion performance. Up to the present day, 20 kHz-range oscillation has been considered a phenomenon that is caused by the first oscillation type (Ionization Oscillation) due to its frequency order.
As regards the features and problems of a micro hall-type electric propulsion in which the size of the propulsion is small, a reduction in weight and a corresponding reduction in launch costs can be achieved, and therefore demand for this type of propulsion in a micro-spacecraft of 100 kg or less is high. A high-specific impulse, small-sized propulsion, with which an increase in payload ratio and a reduction in fuel consumption can be realized, shows promise as a propulsion system for installation in such a micro-spacecraft. Due to their low power consumption and ability to generate thrust semi-continuously over a long time period, hall-type electric propulsions show particular promise in cases where communication satellites having high business needs are subjected to station keeping at a low orbit near Earth. However, a high-performance, small-sized hall-type electric propulsion has not yet been realized.
The reason (problem) why it is difficult to realize this type of propulsion is that when a magnetic pole (material: soft iron) forming a magnetic circuit generated by a magnetic coil installed in the propulsion is overheated to or above a magnetic transformation point, the magnetic susceptibility of the soft iron varies, causing a distortion in the magnetic line of force distribution (initial design). When the magnetic line of force distribution distorts, the acceleration vector of the ions that are accelerated by the electromagnetic field (electromagnetic force) becomes offset, and as a result, the ions collide on the acceleration channel wall surface before being emitted to the exterior of the acceleration channel. This leads not only to the reduction in propulsion efficiency (see Equation (25), to be described below) due to ion loss, but also to sputtering on the acceleration channel wall surface. As a result of this wear, the thickness of the acceleration channel wall surface material (material: ceramic, alumina-type ceramic; 3Al2O3/2SiO2 or boron nitride; BN), which acts as a heat-resistant/insulating wall, decreases locally, leading to a reduction in the heat resistance property against magnetic pole heating by plasma, and consequently a further increase in magnetic pole overheating. This vicious circle worsens as the size of the hall-type electric propulsion decreases. More specifically, as the size decreases, the acceleration channel width narrows, leading to increases in ion sputtering wear on the wall surface and waste heat deterioration. Furthermore, the amount of wall surface loss in the narrow acceleration channel becomes particularly large as micronization advances, and hence it is vital that the aforementioned oscillation phenomenon be solved in order to create a micro hall-type electric propulsion system.